Have you ever seen a garden where you plant new flowers while the old ones are still there? You do that so that the new flowers have some shade and protection from the hot sun, right? Shelterwood cutting is like that, but with trees!
When we want to cut down some trees in a forest, we don't just chop them all down at once. Instead, we do it in stages, just like planting new flowers. We first cut down some older trees to make room for the younger ones to grow. But we keep some of the older trees standing, so they can provide "shelter" and protect the new ones from harsh sunlight.
Gradually, we remove more older trees over the years, until eventually, the younger ones grow big and strong enough to live on their own without any "shelter" from the older trees. And that's how we use shelterwood cutting to take care of the forest!